The present invention comprises novel structures for organic light emitting diodes, and methods of making such diodes. The diodes may be fabricated, for example, on silicon or other opaque substrates.
Light emitting diodes (LEDs) are well known for use in displays and status indicators, and may comprise organic and/or inorganic materials. Inorganic LEDs have the advantage of providing bright and durable point light sources, while organic LEDs (OLEDs) have the advantage of being large area surface emitting light sources. Commercial development of OLEDs has been hampered, however, by their limited lifetime and sensitivity to moisture and oxygen. Typical prior art OLEDs are fabricated on transparent substrates, and radiate through the substrate to produce light. A typical OLED device comprises a glass substrate, a transparent anode layer (e.g., transparent indium-tin oxide (ITO)), a hole transport layer (e.g., a tertiary amine derivative or a hole transporting conducting polymer), an electron transport layer (e.g., tris(8-hydroxyquinoline) aluminum (Alq3) or poly(phenylene vinylene) derivatives, or polyfluorenes), and a metallic cathode layer (e.g., Mg, Ca, or LiF/Al). For most devices, the electron transport layer also serves as the emission layer. Because the cathode layer is opaque, the device radiates only through the transparent substrate. Further, a significant amount of light is generally waveguided laterally and xe2x80x9clost xe2x80x9d through the glass substrate, thus reducing the perceived brightness of the OLED in the forward direction.
It would be desirable to fabricate OLEDs on substrates that allow the reduction of waveguiding losses. One method is to build such a structure on an opaque substrate by inverting the structure so that the cathode is adjacent to the substrate and the transparent (usually ITO) anode is at the top of the device. An advantage of an OLED that does not require a transparent substrate is that the substrate could be made out of silicon. An additional advantage for building the OLED array on silicon is that the substrate can be patterned to define an array of active-matrix current drivers, which could be used to electrically drive the overlying display. Inverted OLEDs have been produced (see, for example, U.S. Pat. No. 5,714,838), but the deposition of ITO onto the organic layers has been found to be difficult (although not impossible) to achieve without degrading the sensitive organic layers. It has also been proposed to use noninverted structures comprising a transparent cathode, but a need remains for innovative OLED structures that can be fabricated on opaque substrates such as silicon. It is an object of the present invention to provide novel OLED structures that can be fabricated on opaque substrates, and particularly on silicon. Furthermore, it is an object of this invention to provide substantially brighter OLEDs, based on opaque substrates, while maintaining ease of fabrication and physical stability of the devices.
The present invention provides OLEDs having superior brightness, which can be formed on opaque substrates such as silicon, and methods for producing such OLEDs. OLEDs according to the invention comprise a reflective electrode on the substrate, an organic active layer that provides electroluminescence, an injection layer, and a transparent electrode. Light from the active layer that is initially directed backward is reflected by the reflective layer to increase the apparent brightness of the diode in the forward direction. The transparent electrode preferably comprises a transparent organic conducting layer.
The OLED may be a double layer diode comprising an organic hole transport layer (e.g., NPB, m-MTDATA, Pani, PEDOT:PSS, or metal-base and metal-free phthalocyanines) and an organic electron transport layer (e.g., Alq3, poly(phenylene vinylene) derivatives, or polyfluorene copolymers), or it may be a single layer diode comprising an organic electroluminescent material with hole and electron transport properties (e.g., poly(phenylene vinylene) derivatives, or polyfluorene copolymers). The OLED may further be inverted (cathode on substrate) or noninverted (anode on substrate).
The invention also includes methods of fabrication of OLEDs, comprising depositing in sequence on a substrate a reflective electrode, an organic active layer, a transparent injection layer, and a transparent organic electrode, to produce an OLED. The steps of depositing the injection layer and the organic electrode do not damage the electronic and/or emissive properties of the underlying layers.